Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 250846
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
246 AM MDT SAT JUL 25 2015

.SHORT TERM...SATURDAY THROUGH MONDAY NIGHT

ONCE AGAIN...A FEW THUNDERSTORMS DEVELOPED ACROSS THE AREA IN
RESPONSE TO THE LEFT FRONT QUADRANT OF A JET STREAK MOVING ACROSS
THE AREA. THERE ARE STILL A FEW ROGUE SHOWERS AND MAYBE A LIGHTNING
STRIKE AROUND. HOWEVER...THE RADAR IS POCATELLO SHOWS THE SHOWERS
ENDING AND WITH THE JET ENERGY MOVING AWAY ALL SHOULD BE QUIET BY
DAYBREAK. AS FOR THE REST OF TODAY...BOTH THE NAM AND GFS ARE RATHER
DRY WITH ONLY ISOLATED SHOWERS AND THUNDERSTORMS OVER MAINLY THE
HIGHER ELEVATIONS AND ADJACENT FOOTHILLS WITH MUCH DRIER AIR PUSHING
OVER THE AREA. OTHERWISE...EXPECT A WARM DAY WITH A SUNNY TO PARTLY
CLOUDY SKY OVER A VAST MAJORITY OF THE AREA. A SOUTHWESTERLY BREEZE
WILL DEVELOP ACROSS THE FAVORED AREAS FROM ROCK SPRINGS TO CASPER
BUT SHOULD NOT REACH CRITICAL FIRE CONDITIONS THOUGH. THESE
THUNDERSTORMS SHOULD END BY AROUND MIDNIGHT WITH THE BALANCE OF THE
NIGHT RAIN FREE.

EXPECT A SIMILAR SITUATION FOR FRIDAY. THE SOUTHERN THREE QUARTERS
OF THE AREA WILL BE COVERED BY VERY LOW PRECIPITABLE WATER
VALUES...UNDER ONE HALF AN INCH WHICH USUALLY PREVENTS CONVECTION IN
MOST INSTANCES. THERE WILL BE ENOUGH MOISTURE FOR ISOLATED STORMS
OVER THE NORTHWEST BUT EVEN HERE CONVECTION IS EXPECTED TO BE OF THE
ISOLATED VARIETY. MEANWHILE...FURTHER EAST THE CONDITION WILL SHIFT
TO FIRE WEATHER AS RELATIVE HUMIDITY WILL FALL CLOSE TO 10 PERCENT
IN SOME AREAS. AT THIS TIME...WINDS DO NOT APPEAR STRONG ENOUGH TO
BRING CRITICAL FIRE WEATHER BUT THIS WILL HAVE TO BE WATCHED.

AS FOR MONDAY A PACIFIC TROUGH WILL APPROACH AND DRAG DOWN A COLD
FRONT THAT WILL PASS THROUGH THE AREA ON MONDAY NIGHT. MODELS HAVE
SOME VERY DRY AIR ACROSS THE MUCH OF THE AREA FOR THE PASSAGE SO AT
THIS TIME WE DO NOT EXPECT A LOT OF CONVECTION WITH THAT MAINLY
LIMITED TO THE NORTH WHERE MOISTURE WILL BE A BIT MORE. BUT EVEN
HERE AGAIN...NOT A LOT OF STORMS ARE EXPECTED. THE BIG THREAT WILL
BE FOR CRITICAL FIRE WEATHER. THE APPROACHING FRONT WILL TIGHTEN THE
PRESSURE GRADIENT AND BRING DECENT SOUTHWESTERLY FLOW ACROSS THE
AREA. THIS WILL COMBINE WITH ANOTHER DAY OF VERY LOW RELATIVE
HUMIDITY THAT COULD DROP DOWN TO SINGLE DIGITS IN SOME AREAS. WITH
EASTERN PORTIONS OF THE COUNTY WARNING AREA NOW HAVING CRITICAL
FUELS...THERE IS A DECENT CHANCE OF SOME FIRE WEATHER HEADLINES FOR
THAT DAY.

.LONG TERM...TUESDAY THROUGH SATURDAY
SYNOPSIS...MUCH COOLER ON TUESDAY BEHIND A COLD FRONT WITH SOME
LINGERING SHOWERS AND ISOLATED THUNDERSTORMS ACROSS THE NORTH.
UPPER LEVEL HIGH PRESSURE WILL STRENGTHEN OVER THE FOUR CORNERS
AREA THROUGH THE REST OF THE WEEK BRINGING A WARMING TREND AND
DRY WEATHER THROUGH FRIDAY. SOME MONSOONAL MOISTURE MAY RETURN TO
THE AREA NEXT WEEKEND.

DISCUSSION...SURFACE COLD FRONT WILL BE WELL EAST OF THE AREA BY
12Z TUESDAY WITH UPPER TROUGH SWINGING ACROSS THE AREA IN THE
MORNING. ECMWF IS SLIGHTLY FASTER THAN GFS WITH UPPER TROUGH
TRANSLATING THE NORTHERN ROCKIES. THOUGH MOISTURE IS
LIMITED...STRONG UPPER TROUGH PASSAGE WILL LIKELY SUPPORT SOME
SHOWER ACTIVITY WITH A FEW THUNDERSTORMS ACROSS THE NORTHERN THIRD
OF THE AREA MAINLY DURING THE MORNING HOURS. LIGHT SNOW
ACCUMULATIONS WILL ALSO BE POSSIBLE ABOVE 10KFT IN THE ABSAROKA
AND BIGHORN RANGES. HIGH TEMPERATURES WILL BE 15F-20F COOLER
BEHIND THE FRONT TUESDAY COMPARED TO PREFRONTAL HOT AND WINDY
CONDITIONS ON MONDAY.

AFTER THIS BRIEF TASTE OF AUTUMN...A MORE MID-SUMMER PATTERN WILL
BECOME ESTABLISHED BY THURSDAY AS UPPER LEVEL HIGH PRESSURE
EXPANDS ACROSS THE SOUTHERN/CENTRAL ROCKIES AND GREAT BASIN
WEDNESDAY THROUGH FRIDAY. MONSOONAL MOISTURE WILL BE SUPPRESSED TO
THE SOUTH THROUGH FRIDAY. THIS PATTERN WILL BRING A RETURN OF
HIGHS IN UPPER 80S TO LOWER 90S ACROSS THE CENTRAL BASINS WITH
MOSTLY 80S IN THE WESTERN VALLEYS. MEDIUM-RANGE GUIDANCE BEGIN TO
DIVERGE OVER THE WEEKEND WITH GFS SHOWING A STRONGER SHORTWAVE
PUSHING ACROSS THE NORTHERN ROCKIES WHILE ECMWF IS MUCH WEAKER
WITH THIS SYSTEM MAINTAINING STRONG RIDGE ACROSS THE AREA...AND
SOME MONSOONAL MOISTURE BEGINNING TO DRIFT NORTH INTO THE AREA.
CLIMATOLOGY FAVORS THE STRONGER FOUR CORNERS HIGH PREVAILING PER
ECMWF IN LATE JULY...EARLY AUGUST SO MORE WEIGHT GIVEN TO ITS
SOLUTION.

&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL AT THE TERMINAL SITES TODAY AND TONIGHT.
SHOWERS OVER THE NORTHWEST AND NORTHERN WY WILL END BY 15Z. ISOLATED
SHOWERS AND STORMS WILL DEVELOP OVER THE NORTHWEST MOUNTAINS AFTER
20Z UNTIL 02Z. SHOWERS AND STORMS WILL ALSO DEVELOP OVER THE BIGHORN
MOUNTAINS AFTER 18Z UNTIL 02Z. SOME OF THOSE STORMS COULD PRODUCE
WIND GUSTS TO 40 KNOTS AND BRIEF HEAVY RAIN. LOCAL MVFR CONDITIONS
COULD OCCUR IN STORMS WITH HEAVY RAIN. THE REST OF THE REGION SHOULD
NOT SEE ANY THUNDERSTORMS TODAY AND TONIGHT.&&

.FIRE WEATHER...

A RIDGE OF HIGH PRESSURE WILL BUILD ACROSS THE AREA TODAY WITH
MAINLY DRY CONDITIONS AND ANY THUNDERSTORMS RESTRICTED TO THE
NORTHWEST AND THE HIGHER ELEVATIONS OF THE BIG HORN RANGE. RELATIVE
HUMIDITY WILL FALL TO THE MIDDLE TEENS IN MANY LOWER ELEVATIONS. A
SOUTHWEST BREEZE WILL DEVELOP BUT WINDS SHOULD REMAIN BELOW
CRITICAL. HOWEVER...NEAR CRITICAL CONDITIONS ARE POSSIBLE FOR SOME
LOCATIONS IN THE EASTERN ZONES. MIXING IS EXPECTED TO BE GOOD WITH
GOOD TO EXCELLENT SMOKE DISPERSAL.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...HATTINGS
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...HATTINGS


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